Many types of security marks or devices have been used to make a variety of objects difficult to replicate or counterfeit. For example, bank notes, passports, driver licenses, and stock certificates contain a variety of security features that thwart copying, such as hard-to-replicate printed watermarks, holograms, micro-print, etc. However, all of these security features are produced using well-known techniques, usually printing, and the security features on each article in the same class of articles are identical. For example, $100 bills of the same design generation have the same watermarks. As long as a counterfeiter can obtain sufficiently advanced equipment, it is possible to reproduce most if not all of these security features. Any newly introduced security feature, such as in bank notes with the same replicated security features, can eventually be defeated.
An optimum security feature for a wide variety of applications should be inexpensive to manufacture but should be extremely difficult to copy. Also, the security feature should be simple to authenticate. The security feature should be able to be directly printed on the article to authenticate (e.g., for passports, licenses, bank notes, etc.) or printed on a label or tag that is securable to an article to authenticate (e.g., for artwork, software boxes, etc.).